Purpose: This study investigated the role of experienced practitioners in promoting pre-service teachers' knowledge construction and social interaction in an online professional learning community. Design/methodology/approach: A repeated measures design with control and experimental groups was adopted. Two practitioners supported pre-service teachers in the experimental group to discuss issues around teaching practice. Social network analysis (SNA) and content analysis (CA) were used in the analytical approach. Findings: CA revealed that the practitioners increased pre-service teachers' levels of knowledge construction and high-cognitive discourse. SNA showed that the practitioners enhanced pre-service teachers' professional ties. Though collaboration in high-level knowledge building occurred predominantly in peers in the same discipline, the presence of the practitioners facilitated the development of more cross-disciplinary ties in the experimental group. Practical implications: The practitioners can be considered as a pedagogical tool to enhance pre-service teachers' engagement in the process of professional learning. This study suggests that in order to enhance the collaboration among pre-service teachers from different departments, the online activity should be designed to enhance interdisciplinary collaboration. Originality/value: The study contributes new knowledge about the ways in which practitioners can enhance the collaboration among pre-service teachers in an online PLC. It also provides insight on how to combine CA and SNA, to examine professional learning.